I. Technical Field
The present invention relates to a silicone boot for a constant velocity universal joint and a constant velocity universal joint.
II. Description of the Related Art
For the purpose of preventing intrusion of foreign matters such as dust into the joint and leakage of grease sealed in the joint, a bellows-like boot is attached to a constant velocity universal joint used for power transmission in automobiles and various industrial machines. The following are known as a material for the boot for a constant velocity universal joint: a silicone material, a chloroprene (CR) material, an ethylene-acrylic rubber (VAMAC) material, a chlorinated polyethylene (CM) material, or the like.
As illustrated in FIG. 7, a boot of this type includes the following: a larger diameter portion 101 fixed to an outer race as an outer joint member of a constant velocity universal joint; a smaller diameter portion 102 fixed to a shaft 108 extending from an inner race as an inner joint member; and a bellows portion 105, which is provided between the larger diameter portion 101 and the smaller diameter portion 102, and has valley portions 103 and peak portions 104 formed alternately with each other. Each of the larger diameter portion 101 and the smaller diameter portion 102 is fixed through attachment of a boot band 109.
In recent years, constant velocity universal joints, in particular, constant velocity universal joints for automobiles have been used in a more severe environment owing to space saving or layout thereof near an exhaust pipe. A demand has grown for higher performance of boots for constant velocity universal joints than that in the past. A silicone material, ethylene-acrylic rubber, or chlorinated polyethylene may be adopted as a countermeasure for thermal damage. In this context, a silicone material is known as the most effective material capable of enduring both the high temperature atmosphere of 140° C. or higher and the low temperature atmosphere of −40° C. or lower (JP 2553098 B, JP 10-299788 A and JP 10-299789 A).
Incidentally, the constant velocity universal joint is provided with a function of being rotated while forming operating angles and is rotated while sliding in the axial direction. The boot for a constant velocity universal joint is deformed to follow the behavior. In accordance with the deformation, adjacent peak portions interfere with each other so as to cause abrasion, and inner surfaces of valley portions and a shaft interfere with each other so as to cause abrasion. Alternatively, a stress is repetitively generated in each of the peak portions and the valley portions so as to cause occurrence of fatigue cracks. The boot may suffer breakage as a result of progression of those. Thus, it is necessary to design the boot so as to be excellent in durability such as resistance to the abrasion and the fatigue.
The shape as illustrated in FIG. 8 is adopted as a shape of the smaller diameter portion 102 of a silicone boot. That is, in a radially outer surface of the smaller diameter portion 102, a fitting groove 110 for attachment of a boot band is provided along a circumferential direction. In the fitting groove 110, both axial end surfaces 117 and 118 each extend in a radial direction from a bottom surface 119 so as to form a right angle between the bottom surface 119. Further, between a shaft attachment portion 111 provided with the fitting groove 110 and one of the peak portions 104 on a smaller diameter side of the bellows portion 105, there is formed a relatively thick coupling portion 120. That is, a straight portion 112 is formed from a radially outer end of the axial end surface 117 on a bellows-portion side of the fitting groove 110 toward the bellows portion. The straight portion 112 and a radially outer surface of the one of the peak portions 104 of the bellows portion 105 are continuous with each other through intermediation of a round portion 113.